Osteoporosis is a condition of the human skeleton that is characterized by deleterious loss, over time, of bone mineral content, particularly calcium. The disease, while most prevalent in women past menopause, is commonly considered an aging disease present in both men and women. Individuals exhibiting osteoporosis are very prone to fractures, most commonly in the wrist, spine and hips. Death rates among men and women due to complications associated with osteoporosis are quite significant, numbering in the tens of thousands a year in North America. Experience has revealed no single measure of bone quality or quantity that is a reliable indicator of fracture risk. For example, microcracks from prior stresses increase risk but are not "visible" in most measurements. Nevertheless, it is widely accepted that the strength of bone (i.e., its resistance to fracture) is roughly proportional to the mass density of the bone mineral and this, in turn, is proportional to the calcium content.
New treatments and therapies have recently been, and are currently being, developed to treat osteoporosis. Two basic approaches to treatment are taken; one relates to intervening in order to reduce the amount of bone loss which accompanies aging, and the other involves replacing lost calcium or increasing calcium content. Early detection of bone mineral loss would be very advantageous, particularly if steps can be taken in the very early stages to slow down calcium depletion. The ability to measure, noninvasively and in situ, bone mineral content is crucial to early detection of osteoporosis and other related skeletal degenerative diseases.
There are several techniques available for measurement of bone mineral content. Computed tomography (CT) involves measurement of X-rays transmitted through the different parts of the anatomy detected by arrays of detectors whereupon cross-sectional images are constructed of internal structures of the body from transmitted X-ray data from which mineral loss data is obtained. Dual energy X-ray absorptometry (DXA) uses a dual energy approach in order to correct for tissue variations and to permit quantification of bone mass.
More specifically, U.S. Pat. No. 5,535,750 issued to Matsui et al. is directed to a method and device for monitoring development of osteoporosis using ultrasonic monitoring of a heel or a knee bone. The method involves measurement of velocity differences of acoustic signals transmitted through the subject bones.
U.S. Pat. No. 5,483,965 issued to Weiner et al. teaches insertion of the heel of a person into a water bath in an apparatus containing ultrasonic transducers to perform densitometry. Acoustic signals are transmitted through the user's foot and a receiver on the other side of the foot detects the signals and measures the transit time and attenuation of a selected frequency thereby obtaining a profile of the bone content.
U.S. Pat. No. 5,335,260 issued to Arnold is directed to a method of quantifying calcium, bone mass and bone mass density via X-ray radiography that involves use of a calibration phantom comprising a material to simulate human tissue. X-rays of sufficient energy and intensity are transmitted through the limb and a detector on the other side of the limb processes the transmitted X-ray data.
U.S. Pat. No. 5,204,888 issued to Tamegai et al. discloses a method for measurement of bone mineral content through irradiating an object with X-rays and measuring the transmitted X-ray intensity. The device uses an X-ray generator that produces X-rays over a continuous spectrum and a detector placed on the other side of the object being probed to measure transmitted X-ray intensities.
U.S. Pat. Nos. 4,510,450 and 4,635,643, both issued to Brown teach use of nuclear magnetic resonance for determining mineral content of bone. U.S. Pat. No. 4,510,450 claims a rotor device which acts as a holder for the assay during the test and U.S. Pat. No. 4,635,643 claims the actual method of probing for mineral content using .sup.31 P NMR.
U.S. Pat. No. 5,521,955 issued to Gohno et al. discloses an apparatus for bone density measurement and non-destructive inspection using a computed tomography (CT) scanner. The method requires scanning a calibration sample produced by mixing a water equivalent material (a material having the same X-ray transmission rate as that of water) with different ratios of a standard material equivalent to bone mineral mass (a material having the same X-ray transmission rate as that of bone mineral mass) and determining the bone density relative to the standard samples.
U.S. Pat. No. 4,829,549 issued to Vogel et al. is directed to a densitometer for predicting osteoporosis by measurement of bone mineral content by transmission of X-rays/gamma rays through the heel bone. A foot holder is provided with a radioactive source holder mounted in the foot holder along with a detector mounted in the foot holder opposite the source holder,
U.S. Pat. No. 5,351,689 issued to MacKenzie teaches a method and apparatus for low dose estimates of bone minerals using gamma ray backscattering. The method disclosed in this patent relies upon measuring the backscattering from bones and comparing the intensities in two areas of the backscatter spectrum. One area, A.sub.1, derives most of its intensity from Rayleigh scattering while the other area, A.sub.2, combines the events from both Rayleigh and Compton scattering. The shape parameter, W=A.sub.1 /A.sub.2, is approximately a linear function of bone mineral content because most of the Rayleigh scattering is due to calcium content of the bone mineral.
A drawback to many of the above-mentioned devices and procedures for measuring bone content is the need for very expensive, large and heavy equipment and in some cases high radiation doses. Such systems, for example the DXA and CT systems require a dedicated centralized location and require attendance by specialized technicians to oversee the scanning process. This results in availability being restricted to medical facilities that are financially well supported.
Therefore, it would be very advantageous to provide an in vivo, low dose, rapid and inexpensive method and device for monitoring bone mineral content that is portable and does not require sophisticated analysis techniques for interpreting the results. Such a device would readily lend itself to large scale use and may be used by any age group for monitoring bone development and would be very useful as a first tool in a program for early detection and prevention of osteoporosis as well as for monitoring the effectiveness of any dietary or pharmaceutical therapeutic program in respect of impact on bone mineral content.